This invention relates generally to a miniature motor for use in audio equipment, video equipment, automotive electrical equipment, etc. and a method of making the same, and more particularly to a miniature motor having a motor case formed into a cross section consisting of two parallel straight lines (parallel planar portions); and two opposing concentric arc segments, and a method of making the same.
FIG. 1 is a perspective view illustrating the essential part of a miniature motor to which this invention is applied. In FIG. 1, reference numeral 1 refers to a motor case made of a metallic material, such as mild steel, and formed into a bottomed hollow cylindrical shape having flat portions 2 constituting parallel planes. Numeral 3 refers to an end bracket made of a resin material, formed into a plate shape, and fitted to an open end of the motor case 1. Numeral 4 refers to a rotating shaft fixedly fitted to the center of an armature (not shown) provided in the motor case 1 and rotatably supported by a bearing 9. On the end bracket 3; there are provided brushes, a bearing, input terminals, etc. (all of them are not shown).
FIG. 2 is a cross-sectional view illustrating the essential part of the miniature motor shown in FIG. 1. Like parts are indicated by like numerals shown in FIG. 1. In FIG. 2, the motor case 1 is formed into a shape having a cross section consisting of two parallel straight lines and two opposing concentric arc segments. Numeral 5 refers to permanent magnets made of ferromagnetic material, such as hard ferrite, formed into an arc segment shape, and fixedly fitted to the cylindrical inside surface of the motor case 1. Numeral 6 refers to an armature rotatably supported in the motor case 1. With the aforementioned construction, as current is fed from the input terminals (not shown) to the armature 6 via brushes (not shown), rotating force is imparted to the armature existing in a magnetic field formed by a pair of the permanent magnets 5 fixedly fitted to the cylindrical inside surface of the motor case 1, causing the armature 6 to rotate, and external equipment to be driven via the rotating shaft 4.
Since the miniature motor having the aforementioned construction has flat portions 2 on the motor case 1, it has an advantage that the distance L between the flat portions 2 and 2 can be made smaller than the outside diameter D of the motor case 1. Thus, the miniature motor having the aforementioned shape can be installed in a space smaller than that needed for the miniature motor of a cylindrical shape.
The outside diameter of the armature 6 must be such a size that intervention with the inside surface of the motor case 1 can be prevented when the thickness of the material for the motor case 1 is constant and the length L between the flat portions 2 and 2 is set to a predetermined length. And, once the outside diameter of the armature 6 is determined, the output torque is limited to a predetermined range, though there may be some differences depending on the magnetic properties of the permanent magnet 5. Consequently, the output torque generated is limited by the fact that the outside diameter of the armature 6 is limited by such a consideration as to prevent intervention with the inside surface of the motor case 1 in a miniature motor having the construction shown in FIG. 2.
To overcome the above problems, a construction in which notched portions 7 are provided on part of the flat portions 2 of the motor case 1, as shown in FIG. 3, has been proposed. A miniature motor of this construction has advantages that the outside diameter of the armature 6 can be increased, and as a result output torque can also be increased accordingly.
In a miniature motor of the construction shown in FIG. 3, however. the mechanical strength of the motor case 1 is naturally be lowered because of the notched portions 7. Furthermore, there is a danger of foreign matter entering inside the motor case 1 from the notched portions 7, leading to a seizure of the armature 6. Moreover, mechanical noise generated inside the motor case 1 may leak to the outside from the notched portions 7. This could result in a remarkable deterioration in acoustic properties when a miniature motor is used to drive audio equipment.